1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More specifically, the present invention is in the field of voltage level shifting in electrical circuits and systems.
2. Background Art
Level shifter circuits are widely used in a variety of modern electronic devices and systems. Many integrated circuit (IC) applications, for example, require translation of one or more signals from one voltage level to another, and thus employ level shifter circuits to translate, or shift, a lower voltage input signal to a higher voltage output, or vice versa. For instance, input/output (IO) pads utilized in an IC may rely on a level shifter circuit to translate a low voltage core signal to a higher voltage level suitable for external signaling.
Level shifting circuits have traditionally been implemented using transistors configured to pull-up and pull-down various circuit nodes in order to perform the voltage translation. For example, in a level shifter circuit configured to translate a lower voltage input to a higher voltage output, the lower input voltage is typically used to drive the pull-down transistors, which must overdrive the pull-up transistors at some stages of the level shifting process. However, as advances in technology result in ever lower core voltage signals, the input signals driving the pull-down transistors may now approach the threshold voltages of those devices. As a result, those pull-down devices may be lightly driven and thus encounter difficulty in overdriving their companion pull-up devices, as is necessary for successful operation of the circuit.
One conventional approach to solving the aforementioned problem is to greatly reduce the size of the pull-up transistors relative to the pull-down devices with which they are paired. For example, it is not uncommon for a pull-up device to be ten or more times smaller than its pull-down paired device. Although the conventional solution can enhance the ability of the pull-down device to overdrive its pull-up companion, that approach typically results in substantial duty cycle distortion because of the relative weakness of the pull-up devices. However, many modern electronic systems require tighter control over duty cycle symmetry than can be achieved using the conventional approach, resulting in degraded performance of those systems when used in conjunction with conventional level shifter circuit receiving low voltage inputs.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing a low voltage input level shifter circuit configured to reduce or eliminate duty cycle distortion.